Output sentence generation apparatus, output sentence generation method, and output sentence generation program

ABSTRACT

An output sentence generation apparatus for automatically generating one output sentence from a plurality of input keywords includes a candidate sentence generator incorporating a learned neural network configured to take in the plurality of keywords and generate a plurality of candidate sentences each including at least some of the plurality of keywords, and an evaluation outputter configured to calculate an overlap ratio for each of the plurality of candidate sentences generated by the candidate sentence generator and increase an evaluation of the candidate sentence with a small overlap ratio to thereby determine an output sentence from the plurality of candidate sentences. The overlap ratio is the number of occurrences of an overlapping word with respect to the number of occurrences of all words included in the corresponding candidate sentence.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2017-39141, filed on Mar. 2, 2017, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present disclosure relates to an output sentence generationapparatus, an output sentence generation method, and an output sentencegeneration program.

In a natural language processing system, a sentence generation techniqueusing a neural network has come to be known (see, for example, JapaneseUnexamined Patent Application Publication No. H01-255966). Recently,sentence generation apparatuses using learned neural networks havebecome known. The learned neural networks, when provided with keywords,generate sentences taking therein a moderate number of the keywords.

SUMMARY

When a neural network learns, with some learning data, it learns thatsentences including overlapping provided keywords are correct sentences.When the learned neural network that learned in this way is used, therehas been a problem that sentences taking therein overlapping providedkeywords are frequently generated. In actual natural sentences, thereare few cases where specific words overlap. Thus, such sentencesincluding overlapping keywords are desirably less frequently generated.However, in order to achieve this purpose, it is necessary to prepare anenormous amount of learning data to improve learning accuracy of theneural network. This requires a large workload. Further, the convenienceof using existing neural networks would be lost.

The present disclosure provides a technique for generating an outputsentence that avoids overlapping keywords to thereby give a more naturalimpression while using a learned neural network.

A first example aspect of the present disclosure is an output sentencegeneration apparatus for automatically generating one output sentencefrom a plurality of input keywords. The output sentence generationapparatus includes: a candidate sentence generator incorporating alearned neural network configured to take in the plurality of keywordsand generate a plurality of candidate sentences each including at leastsome of the plurality of keywords; and an evaluation outputterconfigured to calculate an overlap ratio for each of the plurality ofcandidate sentences generated by the candidate sentence generator andincrease an evaluation of the candidate sentence with a small overlapratio to thereby determine an output sentence from the plurality ofcandidate sentences. The overlap ratio is the number of occurrences ofan overlapping word with respect to the number of occurrences of allwords included in the corresponding candidate sentence.

According to the output sentence generation apparatus configured in thisway, there is no need to readjust the learned neural network. Further,the plurality of candidate sentences output from the learned neuralnetwork are evaluated as to whether they include the overlappingkeywords. Therefore, it is possible to generate the output sentence thatavoids overlapping keywords by simple processing while utilizingexisting resources.

Further, the above evaluation outputter may increase the evaluation ofthe candidate sentence with a small overlap ratio, calculate a keywordratio for each of the plurality of candidate sentences, and increase anevaluation of the candidate sentence with a small keyword ratio tothereby determine the output sentence from the plurality of candidatesentences and output the output sentence. The keyword ratio is thenumber of elements in an intersection of a group of the plurality ofkeywords and a group of all words included in the correspondingcandidate sentence with respect to the number of elements in a union ofthe group of the plurality of keywords and the group of all the wordsincluded in the corresponding candidate sentence. In this way, by makingthe evaluation also in consideration of the keyword ratio, it ispossible to reduce the possibility that a sentence with a small numberof the keywords taken therein may be determined as the output sentence.

In this case, the above candidate sentence generator generates anevaluation score N together with each of the plurality of candidatesentences. The higher a possibility that the candidate sentence might bedetermined as the output sentence, the greater a value of the evaluationscore N becomes. The evaluation outputter calculates, as the overlapratio, P=1-(the number of occurrences of the overlapping word)/(thenumber of occurrences of all the words). The evaluation outputtercalculates, as the keyword ratio, J=(the number of elements in theintersection of the group of the plurality of keywords and the group ofall the words included in the corresponding candidate sentence)/(thenumber of elements in the union of the group of the plurality ofkeywords and the group of all the words included in the correspondingcandidate sentence). The evaluation outputter can output the candidatesentence with a largest value of N×P×J as the output sentence. In thisway, by replacing the evaluation with a specific and simple numericalvalue, it is possible to more quickly determine the output sentence.

In such an evaluation, when the overlap ratio is calculated and when thekeyword ratio is calculated, particles, auxiliary verbs, andconjunctions may be excluded. Particles, auxiliary verbs, andconjunctions among words are heterogeneous as compared with other partsof speech from the viewpoint of the impression that overlapping wordgives to natural sentences. Therefore, in the above method, the words tobe counted as elements are limited to the content words excludingparticles, auxiliary verbs, and conjunctions. Further, in addition toexcluding particles, auxiliary verbs, and conjunctions specific parts ofspeech may be excluded. By excluding specific parts of speech, it ispossible to output highly accurate output sentences according to thepurpose of use and usage status. Moreover, an increase in a calculationspeed can be expected.

A second example aspect of the present disclosure is an output sentencegeneration method for automatically generating one output sentence froma plurality of input keywords. The output sentence generation methodincludes: taking in the plurality of keywords and, using a learnedneural network, generating a plurality of candidate sentences eachincluding at least some of the plurality of keywords; and calculating anoverlap ratio for each of the plurality of candidate sentences generatedin the generating and increasing an evaluation of the candidate sentencewith a small overlap ratio to thereby determine an output sentence fromthe plurality of candidate sentences. The overlap ratio is the number ofoccurrences of an overlapping word with respect to the number ofoccurrences of all words included in the corresponding candidatesentence.

A third example aspect of the present disclosure is an output sentencegeneration program for automatically generating one output sentence froma plurality of input keywords. The output sentence generation programcauses a computer to execute: taking in the plurality of keywords and,using a learned neural network, generating a plurality of candidatesentences each including at least some of the plurality of keywords; andcalculating an overlap ratio for each of the plurality of candidatesentences generated in the generating and increasing an evaluation ofthe candidate sentence with a small overlap ratio to thereby determinean output sentence from the plurality of candidate sentences. Theoverlap ratio being the number of occurrences of an overlapping wordwith respect to the number of occurrences of all words included in thecorresponding candidate sentence. The second and third example aspectscan be expected to achieve the same effects as those of the firstexample aspect.

According to the present disclosure, it is possible to generate anoutput sentence that avoids overlapping keywords to thereby give a morenatural impression while using a learned neural network.

The above and other objects, features and advantages of the presentinvention will become more fully understood from the detaileddescription given hereinbelow and the accompanying drawings which aregiven by way of illustration only, and thus are not to be considered aslimiting the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a functional block diagram of an output sentence generationapparatus according to the embodiment;

FIG. 2 is a diagram showing a specific example from input to output;

FIG. 3 is a flowchart showing an overall processing flow; and

FIG. 4 is a diagram showing a smartphone as an application example of adevice.

DESCRIPTION OF EMBODIMENTS

Hereinafter, although the present disclosure will be described withreference to embodiments of the invention, the present disclosureaccording to claims is not limited to the following embodiment.Moreover, all the components described in the following embodiment arenot necessarily indispensable for means to solve problems.

FIG. 1 is a functional block diagram of an output sentence generationapparatus 100 according to this embodiment. When a plurality of keywordsare provided, the output sentence generation apparatus 100 automaticallygenerates one sentence including at least some of these keywords andoutputs the sentence. For example, the output sentence generationapparatus 100 is incorporated in a humanoid robot that conducts a speechdialogue with a user. When the humanoid robot replies to confirm auser's intention, it passes the keywords extracted from an utterance ofthe user to the output sentence generation apparatus 100, receives thesentence generated as a result thereof, and utters the sentence from aspeaker or the like. Alternatively, a server may function as the outputsentence generation apparatus 100. For example, the server may generatea sentence using the keywords transmitted from a user's terminal via anetwork and send back the sentence to the terminal.

As described above, the output sentence generation apparatus 100 may notinclude an independent structure as hardware. Alternatively, the outputsentence generation apparatus 100 may be incorporated as a part ofanother apparatus. Further, a chip such as an ASIC may be an entity ofthe output sentence generation apparatus 100.

The output sentence generation apparatus 100 described in thisembodiment includes a keyword input unit 110 as an input interface and apresentation unit 140 as an output interface. The output sentencegeneration apparatus 100 further includes a candidate sentencegeneration unit 120 and an evaluation output unit 130, which arecalculation units performing calculations. For example, each of thecandidate sentence generation unit 120 and the evaluation output unit130 may be composed of a CPU as a general-purpose arithmetic processingchip or of a dedicatedly-designed ASIC.

Further, the calculation unit may include, for example, a memory thatstores a language database organized into a corpus or may be configuredto access a language database stored in an external memory. Thecandidate sentence generation unit 120 generates a sentence by referringto the language database.

The keyword input unit 110 is composed of, for example, a microphone ashardware and a speech analysis unit that analyzes an input speech. Inthis case, the keyword input unit 110 extracts keywords by speechanalysis and passes them to the candidate sentence generation unit 120.The keyword input unit 110 may accept a keyword input directly from theuser via a keyboard or the like. When the output sentence generationapparatus 100 is incorporated in another apparatus, the keyword inputunit 110 is an interface that accepts a target sentence or keywords fromthe output sentence generation apparatus 100.

The presentation unit 140 is composed of, for example, a speaker ashardware and a signal conversion unit that converts the generatedsentence into a speech signal. In this case, the presentation unit 140converts one sentence received from the evaluation output unit 130 intoa speech signal and utters the speech signal from the speaker. Thepresentation unit 140 may be a display unit that displays, in text form,one sentence received from the evaluation output unit 130. When theoutput sentence generation apparatus 100 is incorporated in anotherapparatus, the presentation unit 140 is an interface that passes thesentence received from the evaluation output unit 130 to the outputsentence generation apparatus 100.

The candidate sentence generation unit 120 is composed by incorporatingtherein an NN generator 121, which is a learned neural network. Thecandidate sentence generation unit 120 may be configured to be capableof updating the NN generator 121 via a communication interface.

The NN generator 121 takes in the keywords provided from the keywordinput unit 110, generates a plurality of candidate sentences eachincluding at least some of these keywords, and calculates a basic scoreof each of the candidate sentences. The basic score is evaluated by theNN generator 121. The basic score indicates as to how suitable thecandidate sentence is as an output sentence. The basic score is definedin such a way that, for example, between 0 and 1, the closer the basicscore is to 1, the more suitable the candidate sentence is as the outputsentence. In other words, the NN generator 121 makes the evaluation suchthat the closer the calculated value is to 1, the more suitable thecandidate sentence is as the output sentence.

In the neural network implemented as the NN generator 121, modellearning is performed by an external computer. In the model learning,content words excluding pronouns, articles, auxiliary particles,auxiliary verbs, and conjunctions are extracted from correct sentencesprovided as supervised data, one sentence is generated from theextracted content words, and errors in respective layers are repeatedlyupdated so that the generated sentence becomes close to the correctsentence.

The candidate sentence generation unit 120 according to this embodimentcan use various types of known neural networks as the NN generator 121.Examples of the known neural networks that can be used as the NNgenerator 121 are: a sentence generation module by seq2seq using LSTM(see, for example, Ilya Sutskever, Oriol Vinyals, and Quoc V Le,“Sequence to sequence learning with neural networks”, In NIPS. 2014. orKyunghyun Cho, Bart van Merrienboer, Dzmitry Bandanau, and YoshuaBengio, “On the properties of neural machine translation:Encoder-decoder approaches,”In SSST, 2014.), a sentence generationmodule by seq2seq using RNN (see, for example, Liang Lu, Xingxing Zhang,Steve Renais, “On training the recurrent neural network encoder-decoderfor large vocabulary end-to-end speech recognition,” In ICAS SP, 2016.or Iulian V. Serban, Alessandro Sordoni, Yoshua Bengio, Aaron Courvilleand Joelle Pineauy, “Building End-To-End Dialogue Systems UsingGenerative Hierarchical Neural Network Models,”https://arxiv.org/pdf/1507.04808.pdf), a sentence generation module byseq2seq using Bi-directional LSTM (see, for example, Xuezhe Ma, EduardHovy, “End-to-end Sequence Labeling via Bi-directional LSTM-CNNs-CRF,”https://arxiv.org/abs/1603.01354 or Andrej Karpathy, Li Fei-Fei, “DeepVisual-Semantic Alignments for Generating Image Descriptions,”in CVPR,2015.), a sentence generation module disclosed in Japanese UnexaminedPatent Application Publication No. H01-255966, and a sentence generationmodule disclosed in Dusek+, 2016 (see Ondrej Dusek and Filip Jurcicek,“Sequence-to-sequence generation for spoken dialogue via deep syntaxtrees and strings,” In ACL, 2016.)

The candidate sentence generated by the NN generator 121 may use theprovided keyword multiple times. This is because the incorporated neuralnetwork has learned correct sentences including the same word multipletimes during learning. However, in actual natural sentences, there arefew cases where specific words overlap. Thus, such sentences includingoverlapping keywords are desirably less frequently generated. However,in order to achieve this purpose, it is necessary to prepare enormousamount of learning data to improve learning accuracy of the neuralnetwork. This requires a large amount of workloads. Further, theconvenience of using existing neural networks would be lost.

For the above reason, in this embodiment, the output candidate sentenceis further analyzed while the learned neural network is still used inorder to avoid the candidate sentence that uses the overlapping keyword.The evaluation output unit 130 calculates an overlap ratio and a keywordratio to evaluate the candidate sentence. The overlap ratio indicateshow much the content words overlap in each of the plurality of candidatesentences generated by the candidate sentence generation unit 120. Thekeyword ratio indicates how many of the plurality of provided keywordsare adopted in each of the plurality of candidate sentences generated bythe candidate sentence generation unit 120. Then, evaluation output unit130 determines the candidate sentence with the highest evaluation as theoutput sentence and outputs it to the presentation unit 140.Hereinafter, a method for calculating and evaluating the overlap ratioand the keyword ratio will be described below by showing a specificexample.

FIG. 2 is a diagram showing the specific example from input to output.Here, a case is described, in which three content words “commit”, “sin”,and “every” are provided as examples of the keywords. The outputsentence generation apparatus 100 aims to output a natural sentencewhich includes as many of these keywords as possible and also avoidsoverlapping of the same keyword.

The NN generator 121 generates a plurality of sentences from theprovided keywords “commit”, “sin”, and “every”. Among the generatedsentences, three sentences with high basic scores are determined as thecandidate sentences. In this case, suppose that a candidate sentence 1,which is “I commit a sin of a sin.”, a candidate sentence 2, which is “Icommit a rule of a sin.”, and a candidate sentence 3, which is “I commitevery sin.” have been generated. Further, in this example, it is assumedthat the basic score for the three candidate sentences is calculated asN=0.8.

The evaluation output unit 130 receives these three candidate sentences,and the score calculation unit 131 calculates evaluation values. Thescore calculation unit 131 first calculates a penalty coefficient. Thepenalty coefficient is one index of the overlap ratio. The overlap ratiorepresents a ratio of the number of occurrences of the overlappingcontent word with respect to the number of occurrences of all thecontent words included in the candidate sentence. To be more specific,the penalty coefficient P is defined as follows.

P=1-(the number of occurrences of the overlapping content word)/(thenumber of occurrences of all the content words)

As there are three content words included in the candidate sentence 1,which are “sin”, “sin”, and “commit”, the number of occurrences of thecontent words in the candidate sentence 1 is “3”. Further, in thecandidate sentence 1, “sin” appears twice. Thus, the number ofoccurrences of the overlapping content word is “1”. Note that the numberof occurrences when the overlapping identical content word appears n (nis a natural number of two or greater) times in the candidate sentenceis counted as “n-1”. Therefore, the penalty coefficient of the candidatesentence 1 is P=1-1/3=2/3.

As there are three content words included in the candidate sentence 2,which are “sin”, “rule”, and “commit”, the number of occurrences of thecontent words in the candidate sentence 2 is “3”. Further, in thecandidate sentence 2, there is no overlapping content word. Thus, thenumber of occurrences of the overlapping content word is “0”. Therefore,the penalty coefficient of the candidate sentence 2 is P=1-0/3=1.

As there are three content words included in the candidate sentence 3,which are “sin”, “every”, and “commit”, the number of occurrences of thecontent words of the candidate sentence 3 is “3”. Further, in thecandidate sentence 3, there is no overlapping content word.

Thus, the number of occurrences of the overlapping content word is “0”.Therefore, the penalty coefficient of the candidate sentence 3 isP=1-0/3=1.

Then, the score calculation unit 131 calculates a Jaccard coefficient.The Jaccard coefficient is one index of the keyword ratio. The keywordratio represents the number of elements in an intersection of a group ofkeywords and a group of all the content words included in the candidatesentence with respect to the number of elements in a union of the groupof keywords and the group of all the content words included in thecandidate sentence. To be more specific, the Jaccard coefficient isdefined as follows.

J=(the number of elements in the intersection of the group of keywordsand the group of all the content words included in the candidatesentence)/(the number of elements in a union of the group of keywordsand the group of all the content words included in the candidatesentence)

The group of keywords is {“commit” “sin” “every”}, and the group of allthe content words included in candidate sentence 1 is {“sin” “commit”}.The intersection of these groups is {“commit” “sin” }, and the number ofelements is “2”. Likewise, the union of these groups is {“commit” “sin”“every”}, and the number of elements is “3”. Accordingly, the Jaccardcoefficient of the candidate sentence 1 is J=2/3.

Similarly, the group of all the content words included in the candidatesentence 2 is {“sin” “rule” “committed”}. The intersection with thegroup of keywords is {“commit” “sin”}, and the number of elements is“2”. Likewise, the union is {“commit” “sin” “every” “rule”}, and thenumber of elements is “4”. Accordingly, the Jaccard coefficient of thecandidate sentence 2 is J=2/4=1/2.

Similarly, the group of all the content words included in the candidatesentence 3 is {“sin” “every” “commit”}. The intersection with the groupof keywords is {“commit” “sin” “every”}, and the number of elements is“3”. Likewise, the union of these groups is {“commit” “sin” “every”},and the number of elements is “3”. Accordingly, the Jaccard coefficientof the candidate sentence 3 is J=3/3=1.

The determination unit 132 receives the P value and the J value of eachcandidate sentence calculated by the score calculation unit 131,calculates N×P×J for each candidate sentence, and determines thecalculated value as a final score. Specifically, the final score of thecandidate sentence 1 is 0.36, the final score of the candidate sentence2 is 0.40, and the final score of the candidate sentence 3 is 0.80.Accordingly, the determination unit 132 determines the candidatesentence 3 with the highest score as the output sentence.

Note that particles, auxiliary verbs, and conjunctions among words areheterogeneous as compared with other parts of speech from the viewpointof the impression that overlapping word gives to natural sentences.Therefore, in the above method, the words to be counted, in each of thecandidate sentences, as elements are limited to the content wordsexcluding pronouns, articles, particles, auxiliary verbs, andconjunctions. However, all words including pronouns, articles,particles, auxiliary verbs, and conjunctions may be counted according tothe purpose. Conversely, even among content words, certain parts ofspeech such as adverbs and adjectives may be excluded. In any case, anappropriate adjustment may be made according to the purpose of use andusage status of the output sentence generation apparatus 100. By makingan adjustment in this way, it is possible to output highly accurateoutput sentences according to the purpose of use and usage status.Moreover, as the calculation is simplified, an improvement in thecalculation speed can be expected.

In the above method, the penalty coefficient is used as the overlapratio. However, any specific calculation formula may be used as long asit is a coefficient that increases the evaluation of candidate sentenceswith small overlap ratios. Similarly, in the above method, the Jaccardcoefficient is used as the keyword ratio. However, any specificcalculation formula may be used as long as it is a coefficient thatincreases the evaluation of candidate sentences with large keywordratios.

Moreover, in the above example, it is described that the basic score isthe same for the candidate sentences in order to clarify the influenceof the penalty coefficient and the Jaccard coefficient on the score.However, the actual NN generator 121 may give different basic scores.Therefore, in regard to a plurality of candidate sentences having thesame product of the penalty coefficient and the Jaccard coefficient, theselection of whether they are determined as the output sentence is madeaccording to the magnitude of the basic scores of the candidatesentences.

In the above example, both the overlap ratio and the keyword ratio areconsidered. However, in terms of avoiding candidate sentences includingoverlapping provided keywords, only the overlap ratio may be considered.In that case, the basic score may be multiplied by the penaltycoefficient to obtain the final score.

Next, an overall processing flow of the output sentence generationapparatus 100 will be described. FIG. 3 is a flowchart showing theoverall processing flow. The illustrated processing flow is achieved byexecuting an output sentence generation program read out from a memory.

In Step S101, the keyword input unit 110 accepts an input of a pluralityof keywords. Depending on the configuration of the apparatus, asdescribed above, the keyword input unit 110 may extract keywords from anutterance of the user or may directly accept the keywords from akeyboard or the like.

In the subsequent Step S102, the candidate sentence generation unit 120generates a plurality of candidate sentences and respective basic scoresand outputs them. More specifically, as described above, the NNgenerator 121, which is the learned neural network, performs theprocessing.

In Step S103, the evaluation output unit 130 receives the plurality ofcandidate sentences and respective basic scores, and the scorecalculation unit 131 calculates a corrected score of each candidatesentence. The corrected score is a score obtained by correcting thebasic score. In the example described with reference to FIG. 2, thecorrected score is obtained by multiplying the basic score by thepenalty coefficient and the Jaccard coefficient

In Step S104, in the evaluation output unit 130, the determination unit132 compares the corrected score of the plurality of candidate sentencesto one another, determines the candidate sentence with the highestcorrection score as the output sentence, and passes the output sentenceto the presentation unit 140. Depending on the configuration of theapparatus, as described above, the presentation unit 140 utters thedetermined sentence from a speaker or displays it on a display panel ina text sentence. When the presentation of the output sentence iscompleted, a series of processing is ended.

FIG. 4 is a diagram showing a smartphone 700 as an application exampleof a device. As shown in FIG. 4, the output sentence generationapparatus 100 can be incorporated in a smartphone. The smartphone 700includes a display unit 710, a microphone 711, and a speaker 712. Themicrophone 711 functions as a part of the keyword input unit 110. Thedisplay unit 710 and the speaker 712 function as a part of thepresentation unit 140.

The presentation unit 140 may display, for example, a character 800representing a robot in CG on the display unit 710. The character 800has a head and a body like a dialog robot. The presentation unit 140 canexpress actions by animation in accordance with an utterance of theoutput sentence. The output sentence may be uttered from the speaker712. Additionally or alternatively, a balloon 810 may be displayed onthe display unit 710 and the output sentence may be informed to the userin text form.

The program can be stored and provided to a computer using any type ofnon-transitory computer readable media. Non-transitory computer readablemedia include any type of tangible storage media. Examples ofnon-transitory computer readable media include magnetic storage media(such as floppy disks, magnetic tapes, hard disk drives, etc.), opticalmagnetic storage media (e.g. magneto-optical disks), CD-ROM (compactdisc read only memory), CD-R (compact disc recordable), CD-R/W (compactdisc rewritable), and semiconductor memories (such as mask ROM, PROM(programmable ROM), EPROM (erasable PROM), flash ROM, RAM (random accessmemory), etc.). The program may be provided to a computer using any typeof transitory computer readable media. Examples of transitory computerreadable media include electric signals, optical signals, andelectromagnetic waves. Transitory computer readable media can providethe program to a computer via a wired communication line (e.g. electricwires, and optical fibers) or a wireless communication line.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

What is claimed is:
 1. An output sentence generation apparatus forautomatically generating one output sentence from a plurality of inputkeywords, the output sentence generation apparatus comprising: circuitryconfigured to: take in the plurality of keywords; generate, using alearned neural network, a plurality of candidate sentences eachincluding at least some of the plurality of keywords; calculate anoverlap ratio for each of the generated plurality of candidatesentences; and increase an evaluation of a candidate sentence with asmall overlap ratio to thereby determine an output sentence from theplurality of candidate sentences, wherein the overlap ratio is a numberof occurrences of an overlapping word in a respective candidate sentencewith respect to a number of occurrences of all words included in therespective candidate sentence, and the overlapping word is a first wordin the respective candidate sentence that is the same as a second wordin the respective candidate sentence.
 2. The output sentence generationapparatus according to claim 1, wherein the circuitry is configured to:calculate a keyword ratio for each of the plurality of candidatesentences; increase an evaluation of a candidate sentence with a smallkeyword ratio to thereby determine the output sentence from theplurality of candidate sentences; and output the output sentence,wherein the keyword ratio is a number of elements in an intersection ofa group of the plurality of keywords and a group of all the wordsincluded in the respective candidate sentence with respect to a numberof elements in a union of the group of the plurality of keywords and thegroup of all the words included in the respective candidate sentence. 3.The output sentence generation apparatus according to claim 2, whereinthe circuitry is configured to: generate an evaluation score N togetherwith each of the plurality of candidate sentences, where the higher apossibility that one of the a plurality of candidate sentences might bedetermined as the output sentence, the greater a value of the evaluationscore N becomes, calculate, as the overlap ratio, P=1−(the number ofoccurrences of the overlapping word)(the number of occurrences of allthe words), calculate, as the keyword ratio, J=(the number of elementsin the intersection of the group of the plurality of keywords and thegroup of all the words included in the respective candidatesentence)/(the number of elements in the union of the group of theplurality of keywords and the group of all the words included in therespective candidate sentence), and output a candidate sentence with alargest value of N+P+J as the output sentence.
 4. The output sentencegeneration apparatus according to claim 2, wherein when the overlapratio is calculated and when the keyword ratio is calculated, particles,auxiliary verbs, and conjunctions are excluded.
 5. The output sentencegeneration apparatus according to claim 4, wherein when the overlapratio is calculated and when the keyword ratio is calculated, specificparts of speech are excluded.
 6. An output sentence generation methodfor automatically generating one output sentence from a plurality ofinput keywords, the output sentence generation method comprising: takingin the plurality of keywords; generating, using a learned neuralnetwork, a plurality of candidate sentences each including at least someof the plurality of keywords; calculating an overlap ratio for each ofthe generated plurality of candidate sentences; and increasing anevaluation of a candidate sentence with a small overlap ratio to therebydetermine an output sentence from the plurality of candidate sentences,wherein the overlap ratio is a number of occurrences of an overlappingword in a respective candidate sentence with respect to a number ofoccurrences of all words included in the respective candidate sentence,and the overlapping word is a first word in the respective candidatesentence that is the same as a second word in the respective candidatesentence.
 7. The output sentence generation method according to claim 6,further comprising: calculating a keyword ratio for each of theplurality of candidate sentences; increasing an evaluation of acandidate sentence with a small keyword ratio to thereby determine theoutput sentence from the plurality of candidate sentences; andoutputting the output sentence, wherein the keyword ratio is a number ofelements in an intersection of a group of the plurality of keywords anda group of all the words included in the respective candidate sentencewith respect to a number of elements in a union of the group of theplurality of keywords and the group of all the words included in therespective candidate sentence.
 8. The output sentence generation methodaccording to claim 7, further comprising: generating an evaluation scoreN together with each of the plurality of candidate sentences, where thehigher a possibility that one of the a plurality of candidate sentencesmight be determined as the output sentence, the greater a value of theevaluation score N becomes, calculating, as the overlap ratio, P=1−(thenumber of occurrences of the overlapping word)(the number of occurrencesof all the words), calculating, as the keyword ratio, J=(the number ofelements in the intersection of the group of the plurality of keywordsand the group of all the words included in the respective candidatesentence)(the number of elements in the union of the group of theplurality of keywords and the group of all the words included in therespective candidate sentence), and outputting a candidate sentence witha largest value of N+P+J as the output sentence.
 9. The output sentencegeneration method according to claim 7, wherein when the overlap ratiois calculated and when the keyword ratio is calculated, particles,auxiliary verbs, and conjunctions are excluded.
 10. The output sentencegeneration method according to claim 9, wherein when the overlap ratiois calculated and when the keyword ratio is calculated, specific partsof speech are excluded.
 11. A non-transitory computer readable mediumstoring an output sentence generation program for automaticallygenerating one output sentence from a plurality of input keywords, theoutput sentence generation program causing a computer to execute: takingin the plurality of keywords; generating, using a learned neuralnetwork, a plurality of candidate sentences each including at least someof the plurality of keywords; calculating an overlap ratio for each ofthe generated plurality of candidate sentences; and increasing anevaluation of a candidate sentence with a small overlap ratio to therebydetermine an output sentence from the plurality of candidate sentences,wherein the overlap ratio is a number of occurrences of an overlappingword in a respective candidate sentence with respect to a number ofoccurrences of all words included in the respective candidate sentence,and the overlapping word is a first word in the respective candidatesentence that is the same as a second word in the respective candidatesentence.
 12. The non-transitory computer readable medium according toclaim 11, the output sentence generation program causing the computer toexecute: calculating a keyword ratio for each of the plurality ofcandidate sentences; increasing an evaluation of a candidate sentencewith a small keyword ratio to thereby determine the output sentence fromthe plurality of candidate sentences; and outputting the outputsentence, wherein the keyword ratio is a number of elements in anintersection of a group of the plurality of keywords and a group of allthe words included in the respective candidate sentence with respect toa number of elements in a union of the group of the plurality ofkeywords and the group of all the words included in the respectivecandidate sentence.
 13. The non-transitory computer readable mediumaccording to claim 12, the output sentence generation program causingthe computer to execute: generating an evaluation score N together witheach of the plurality of candidate sentences, where the higher apossibility that one of the a plurality of candidate sentences might bedetermined as the output sentence, the greater a value of the evaluationscore N becomes, calculating, as the overlap ratio, P=1−(the number ofoccurrences of the overlapping word)(the number of occurrences of allthe words), calculating, as the keyword ratio, J=(the number of elementsin the intersection of the group of the plurality of keywords and thegroup of all the words included in the respective candidatesentence)(the number of elements in the union of the group of theplurality of keywords and the group of all the words included in therespective candidate sentence), and outputting a candidate sentence witha largest value of N+P+J as the output sentence.
 14. The non-transitorycomputer readable medium according to claim 12, wherein when the overlapratio is calculated and when the keyword ratio is calculated, particles,auxiliary verbs, and conjunctions are excluded.
 15. The non-transitorycomputer readable medium according to claim 14, wherein when the overlapratio is calculated and when the keyword ratio is calculated, specificparts of speech are excluded.